math_state_restore and kernel_fpu_end disable interrupts?

From: Nate Eldredge
Date: Sat Jan 18 2014 - 01:21:18 EST

In trying to track down a bug (see below), I noticed that math_state_restore() in arch/x86/kernel/traps.c appears to unconditionally disable interrupts when called. Is this intended behavior or a bug?

The bug in question is triggered by dumping core on an ecryptfs file system when aesni-intel is loaded. (See for the original report.) The symptom is that __find_get_block() gets called with interrupts disabled, causing a BUG(). I tried to find where interrupts were getting disabled and wound up in aes_set_key_common() in arch/x86/crypto/aesni-intel_glue.c. It calls aesni_set_key(), and since that uses the FPU, it wraps it in kernel_fpu_begin()/kernel_fpu_end(). But kernel_fpu_end() calls math_state_restore() which disables interrupts. I've verified that interrupts are still enabled just before the call to kernel_fpu_end().

math_state_restore() does:


with the result that interrupts are disabled when it finishes, even if they were enabled to begin with. That looks strange to me; are we sure it shouldn't just save and restore the interrupt flag? Or are we not supposed to call it with interrupts enabled?

Given the intimidating comment preceding math_state_restore() ("Don't touch unless you *really* know how it works"), it's entirely possible I am missing something...

Any suggestions appreciated. Thanks!

Nate Eldredge

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